Previously, photolithography on areas larger than 8".times.8" has been performed using one of three general methods:
(1) Contact or Proximity Printing A mask with the desired pattern is positioned right on top of the substrate (or some 5 to 30 microns higher, in the case of proximity exposure), and collimated light is directed through this mask onto the substrate. PA1 (2) One-Dimensional Scanning Projection Exposure A slit-shaped exposure field is scanned across the mask, and the pattern is transferred by scanning the mask and the substrate with this exposure field. The idea is to avoid any stitching procedures; therefore, the exposure field must be at least as long as the shorter dimension of the substrate. PA1 (3) Step-and-Repeat The substrate is exposed in small rectangular portions, one at a time. Care must be taken that in the transition regions between these portions, no glitches in the alignment (stitching errors) or the exposure intensity (butting error) occur. Large areas are exposed by repetitive use of one or several small masks.
One-dimensional scanning and step-and-repeat operations are inherently expensive. Step-and-repeat needs very high precision stages, which travel over the entire substrate area. This results in a need for laser interferometry, which in turn becomes increasingly costly if applied to long two-dimensional travels. The scanning dimension requires very sophisticated optics, which must yield large field images free of distortions. This drives costs for intermediate panel sizes to very high levels and makes exposure of very large panels (e.g. 500.times.500 mm) virtually impossible.
Proximity printing, on the other hand, is much more cost efficient. However, the depth of field achievable is limited by the collimation angle of the exposure light. Moreover, Fresnel diffraction produces traces of stray light at any edges of the exposed areas. With photosensitive dielectrics requiring high exposure energies and working in the negative mode, such stray light produces thin skins on top of circuit features (lines, holes, etc.), with consequent serious processing problems. Contact printing can reduce such problems, but here the artwork (mask) is subject to substantial degradation during use, as photoresist or particles from the substrates tend to contaminate and/or scratch the mask. For large areas, the survival chances decrease dramatically, and the mask costs increase, while the process yields suffer significantly.
Alternatively, inexpensive masks made from mylar foil are used for printed circuit board exposures. These can be exchanged frequently at low cost, but they lack the dimensional stability for fine line patterning.
In short, a cost-performance gap exists between low cost, low quality contact printing and high performance, high cost exposure equipment. It has been proposed, on paper, to fill this cost-performance gap; however, to the best of applicant's knowledge, this gap has not as yet been filled with something that will actually work satisfactorily and efficiently in the production of quality components.